[C++ Patch] Replace a few more error + error with error + inform

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

while looking into some bugs (eg, c++/71464) I noticed a few more of 
those consecutive errors, which I propose to adjust per the below 
patchlet. The second case in add_method is a bit tricky because in 
principle we'd really like to be more specific (eg, clang talks about 
constructors which cannot be redeclared, member functions which cannot 
be redeclared and many more) and avoid verbose diagnostic, but in the 
below I only try to avoid emitting error + error... Tested x86_64-linux.

Thanks, Paolo.

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